The manufacture of foamed thermoplastic materials by heating the pressurized thermoplastic which is mixed with a foaming agent and then extruding the thermoplastic material to a low pressure zone, is well known and widely used, particularly in making polystyrene foams. Commonly, the foamed material is extruded into a tube which is drawn over the cylindrical surface of a cooling drum and slit to form sheet stock. The sheet stock can then be thermofoamed into articles such as cups, trays and egg cartons. A variety of organic blowing agents including hydrocarbons such as propane, butane, pentane and isopentane as well as halogenated hydrocarbons such as the Freons are commonly used. These foaming agents are released to the atmosphere during the production of the foams and cause pollution. The hydrocarbon blowing agents also create a potential fire hazard. Moreover, the organic blowing agents are costly compared to atmospheric gases. Accordingly, in U.S. Pat. No. 4,424,287 there is disclosed a process in which carbon dioxide is substituted for part of the organic blowing agent thereby mitigating the problems of pollution and fire hazard, and reducing costs. In accordance with this invention, the same advantages are obtained by replacing some of the organic blowing agents with water instead of with carbon dioxide. Moreover, additional advantages are obtained in the production of foam sheets.
U.S. Pat. No. 4,455,272 discloses the formation of extruded polystyrene foams using a mixture of a physical blowing agent such as a hydrocarbon and water to produce foams with an unusual cellular structure. However, this patent does not disclose the method of forming extruded foamed sheets having a smooth, lustrous, drum side skin as produced in accordance with this invention. U.S. Pat. No. 4,455,272 is incorporated herein by reference, in entirety.